Chromium plating solutions have been in widespread commercial use for applying protective and decorative platings to metal substrates. For the most part, commercial chromium plating solutions heretofore used have employed hexavalent chromium derived from compounds such as chromic acid, for example, as the source of the chromium constituent. Such hexavalent chromium electroplating solutions have long been characterized as having limited covering power and excessive gassing particularly around apertures in the parts being plated which can result in incomplete coverage. Additionally such hexavlent chromium plating solutions are quite sensitive to current interruptions resulting in a so called "whitewashing" of the deposit.
Trivalent chromium electroplating baths on the other hand have excellent throwing power and the trivalent chromium plating produced is substantially unaffected by current interruptions during the plating cycle. These factors, coupled with the fact that trivalent chromium compounds are much less toxic than hexavalent chromium compounds have provided impetus for the development of improved trivalent chromium plating baths which achieve the benefits of plating deposits derived from hexavalent chromium plating baths while at the same time overcoming other problems heretofore associated with the trivalent chromium plating system. One such problem relates to a progressive reduction in the plating rate during continued use of a trivalent chromium plating bath due to the progressive increase in the concentration of hexavalent chromium formed interfering in the efficiency of the covering power of the bath.
The solution and process of the present invention overcomes certain disadvantages and problems associated with prior art trivalent chromium electroplating solutions in providing improved reducing agents for minimizing and controlling the concentration of hexavalent chromium in the plating solution whereby plating efficiency and throwing power are maintained at optimum levels over prolonged periods of use.